The present invention relates to a method of preventing negative charge build up on a substrate being implanted with positive ions and in particular to ion implantation apparatus including features for performing such a method.
In the manufacture of semiconductor devices, a popular processing technology is ion beam implantation whereby a beam of desired impurity ions is directed at the semiconductor substrate so that the ions become implanted in the substrate to create regions of a desired conductivity type, for example. Some regions of the substrate being implanted will be electrically insulated from the main body of the semiconductor substrate. As a result, during implantation with ions, such regions become progressively charged. Usually the implanted ions are positive so that positive charge build up on the substrate surface occurs. If this charge build up exceeds the breakdown voltage of insulated layers and regions in the substrate, damage is caused.
There is thus a known requirement to neutralise charge build up on the surface of substrates during ion implantation, and in particular to neutralise positive charge build up.
Known techniques for neutralising this positive charge build up involve flooding the region of the ion beam immediately in front of the substrate with low energy electrons. Any positive charge build up on the substrate tends to attract these electrons to the substrate surface to discharge the surface. Examples of electron flood apparatus, also known as plasma flood guns, are disclosed in U.S. Pat. No. 5,089,710 and U.S. Pat. No. 5,399,871.
In order to be effective the electron flood systems used for substrate neutralisation must produce relatively low energy electrons, which can then readily be attracted to the substrate by relatively small positive charges arising on the substrate. Further, excessively energetic electrons in front of the substrate can impinge upon and stick to the substrate surface even though any positive charge on the substrate surface has already been fully discharged, and thereby build up an excess negative electrostatic charge on the surface. The degree to which a negative electrostatic charge can accumulate on the substrate surface is related to the energy of the electrons in the region in front of the substrate.
Known electron flood systems are designed to ensure that the electrons flooded into the beam in front of the substrate have desirably low energies, typically less than or equal to 5 eV. However, even when the flood electrons have very low energies, excessive negative charge build up on substrate surfaces has been observed during implantation with positive ion beams.
It is known that, during implantation with positive ions, secondary electrons are emitted from the substrate. Reference may be made to the article entitled xe2x80x9cUptime Improvements by Using a Faraday with Magnetic Electron Suppressionxe2x80x9d by Lundquist et al, published in Ion Implantation Technologyxe2x80x9494, pages 702 to 705, by Elsevier Science B.V. This article discusses the need to ensure such secondary electrons emitted from the substrate are captured by a Faraday body located in front of the substrate, where such a Faraday body is used for beam current measurement. For this purpose, the article proposes a magnetic electron suppression device to be located upstream (relative to the beam direction) of the Faraday body. This suppression device is intended to prevent both electrons travelling with the ion beam from entering the Faraday body, and secondary electrons emitted from the substrate from leaving the Faraday body. According to the article, the magnetic field gradient produced by the suppression device at the upstream end of the Faraday body behaves in the same fashion as a magnetic bottle to reflect the electrons back in the direction from which they came. The purpose of the magnetic suppression device is to reflect back all electrons which would otherwise escape from the Faraday body. FIG. 1 in the article shows that a PFG/EFG (Plasma Flood Gun/Electron Flood Gun) is located, between the substrate and the magnetic electron suppression device, to flood electrons into the Faraday body for the purpose of ensuring neutralisation of positive charge build up on the substrate.
The described system also has an array of alternating magnetic poles around the edge of the Faraday body immediately adjacent the substrate or wafer. These provide xe2x80x9cshort range magnetic suppressionxe2x80x9d intended xe2x80x9cto keep electrons from escaping in the gap between the spinning disc (carrying the wafer) and the Faraday. The fields from these magnets within the Faraday are also lowxe2x80x9d (page 704, bottom of the left hand column). Clearly, the short range magnetic suppression between the Faraday body and the wafer itself does not function to prevent electrons from the flood gun diffusing to prevent positive charge build up on the wafer, or to prevent secondary electrons emitted from the wafer from entering the Faraday body.
With the arrangement disclosed in the above article, it is clear that secondary electrons emitted from the substrate enter the Faraday body, so that a population of such electrons may exist in this region. The present invention is based on the understanding that the energy distribution of secondary electrons emitted from a substrate during positive ion implantation may have a peak at around 15 eV with an energy tail extending to over 50 eV. Such secondary electrons experiencing multiple collisions in the beam plasma in front of the substrate may be redirected towards the substrate and extend that kinetic energy in impacting the substrate. This can have the effect of producing negative charge build up on regions of the substrate which are insulated (at least for current to discharge a negative electrostatic charge) from the bulk of the substrate material. The resulting negative charge build up can reach the value equivalent to the highest energy of electrons in the region in front of the substrate (say 50 volts) which would be easily sufficient to cause damage to regions of the substrate.
The present invention provides ion implantation apparatus having an evacuated housing and, contained in said housing,
a) a holder for holding a substrate for implantation;
b) a source of positive ions for implanting in said substrate;
c) a beam of said ions being formed which is directed at said substrate;
d) an electron flood source to supply low energy electrons to the beam in front of said substrate for neutralising positive charge build up on said substrate;
e) a magnetic filter between said substrate holder and said electron flood source providing a magnetic field extending across the beam in front of said substrate holder, said field having a strength selected to deflect out of the region containing the beam secondary electrons emitted from the substrate with energies above 15 eV, but to allow lower energy electrons including those supplied by said flood source to diffuse across said field without being deflected out of said beam region; and
f) a conductive element located out of said beam region to be impacted by and absorb said deflected secondary electrons.
The invention thus provides apparatus which absorbs relatively higher energy secondary electrons emitted from the substrate without impeding the diffusion of the necessary low energy electrons from the flood system to the substrate for neutralisation of positive charge build up. By reducing the number of higher energy secondary electrons in the region immediately in front of the substrate, excessive negative charge build up on the substrate is prevented.
Conveniently said magnetic filter provides said magnetic field with a field strength selected to deflect secondary electrons with energies of above 10 eV and more preferably 5 eV to impact said conductive element. The magnetic filter may include a pair of opposite magnetic poles located on opposite sides of said beam region.
Conveniently, said conductive element comprises a conductive tube surrounding said beam in front of said substrate holder. Thus, the conductive tube may comprise a Faraday body used for measuring the beam current for dosimetry purposes. Where beam current measurements are made in other ways, the conductive tube may still be provided for the purpose of confining the low energy charge neutralisation electrons in the beam region in front of the substrate.
The electron flood source is preferably located to supply said low energy electrons into said conductive tube.
The magnetic filter may be located adjacent to said substrate holder. In the region of said magnetic field, the population density of the higher energy electrons can be much reduced, with a concomitant reduction in the density of these electrons immediately in front of the substrate. The closer the magnetic field is to the substrate, the more effectively the population density of the higher energy electrons immediately in front of the substrate is reduced.
The invention also provides a method of implanting ions in a substrate comprising,
holding the substrate in an evacuated housing;
directing a beam of desired positive ions at the substrate for implanting therein;
flooding the beam in front of the substrate with low energy electrons from an electron flood source to neutralise positive charge build up on the substrate;
applying a magnetic filtering field extending across the beam between the substrate and the electron flood source, said field having a strength selected to deflect out of the region containing the ion beam secondary electrons emitted from the substrate with energies above 15 eV but to allow lower energy electrons, including those from the flood source, to diffuse across said magnetic filtering field without being deflected out of said beam region,
and absorbing said second electrons deflected by said magnetic filtering field on a conductive element located out of said beam region. The magnetic filtering field may be a dipole field, but higher order fields such as quadrupole fields may also be used, provided sufficient field strength is obtained completely across the region containing the beam in front of the substrate so that all secondary electrons emitted from the substrate will experience the magnetic field sufficiently to be deflected out of the beam region.
The invention also provides a method of preventing negative charge build up on a substrate being implanted with positive ions comprising absorbing higher energy secondary electrons emitted from the substrate at a location proximate said substrate, said higher energy secondary electrons being those emitted with energies above a predetermined value, whereby the population density of said higher energy secondary electron immediately in front of said substrate is reduced.